The present invention relates to a process for the manufacture of polycrystalline garnet containing aluminium and/or gallium and/or indium and at least one element taken from the group comprising the rare earths and yttrium, the garnet obtained, and the corresponding monocrystals. More especially and preferably, it relates to a process for the manufacture of gadolinium gallium garnet (Gd.sub.3 Ga.sub.5 O.sub.12) and to the corresponding monocrystal.
The use of monocrystals having a garnet structure, in particular as substrates for the manufacture of magnetic bubble memory devices, is well known. For this use, the monocrystals of garnet are essentially manufactured by Czochralski drawing of the polycrystalline mixed oxides having the garnet structure, which are hereafter referred to as polycrystalline garnets.
At the present time, polycrystalline garnets are prepared in accordance with three processes.
The first of these processes, described, for example, in Journal of Crystal Growth, 12, pp. 3-8 (1972), consists in following a procedure in which the various oxides forming part of the composition of the garnet are mixed in accordance with the following steps:
the various oxides forming part of the composition of the garnet are calcined separately;
the amounts fixed by the formula of the garnet for the various oxides are weighed;
the oxides are mixed mechanically and the mixture obtained is compacted by compression;
the compacted mixture is introduced into the drawing crucible; and
the mixture is melted.
This process exhibits essentially two disadvantages. On the one hand, during the heating of the mixture, it is impossible to characterize the garnet structure of the polycrystalline mixture, the appearance of which depends on the temperature increase program used; on the other hand, because of the evaporation of certain subspecies, the composition of the bath at the moment of drawing, as a function of its composition at ambient temperature, can only be known from empirical relationships.
The second of these processes (described, for example, in German Pat. No. 2,615,554) consists in following a procedure in which the various oxides forming part of the composition of the garnet are mixed and then calcined so as to obtain the garnet structure prior to the melting operation. The calcination conditions (temperature and time) are determined as a function of the kinetics of solid-solid reactions, which themselves depend essentially on the physical properties of the oxides used and on the proportions of impurities therein.
This second prior art process is the most commonly used process for the preparation of polycrystalline garnets. Compared with the first prior art process, it permits a better knowledge of the composition of the bath at the moment of drawing, because of the possibility of characterizing the garnet structure. However, this process exhibits the disadvantage that it generally requires lengthy calcinations which, as has been demonstrated, do not always result in complete reactions between the oxides. For example, in the case of a Gd.sub.3 Ga.sub.5 O.sub.12 garnet, calcination of a mixture of 3 mols of Gd.sub.2 O.sub.3 to 5 mols of Ga.sub.2 O.sub.3 for 100 hours at 1350.degree. C. results in a mixture in which 10 percent by weight of the Ga.sub.2 O.sub.3 has not reacted.
The third of these processes, described, for example, in Journal of Crystal Growth, 19, pp. 204-208 (1979), consists in thermally decomposing a dissolved mixture of the salts (carbonates, nitrates, chlorides, ammonium salts, sulphates, and the like) of the various elements constituting the garnet, and then subjecting the mixture to a heat treatment until the garnet structure is obtained.
The main disadvantage of this third prior art process is that it leads to impure garnets which are contaminated with the impurities resulting from incomplete decomposition reactions (for example, the presence of rare earth oxychlorides).
Furthermore, applicants have developed a process for the manufacture of a polycrystalline mixed oxide having the garnet structure, which overcomes the disadvantages of the earlier processes and exhibits numerous advantages. This process consists in precipitating the co-hydroxides of the cations forming part of the composition of the garnet. This process is described in the commonly assigned, concurrently filed United States application, Ser. No. 204,896, filed Nov. 7, 1980 entitled: "Process for the Manufacture of Polycrystalline Garnet, the Polycrystalline Garnet and the Corresponding Monocrystal". It comprises the following steps:
(1) a solution containing the salts of the constituent cations of the garnet, in the proportions corresponding to the composition of the latter, is prepared; PA0 (2) the corresponding hydroxides are co-precipitated by a base in order to obtain a co-hydroxide; PA0 (3) the co-hydroxide is left to age; PA0 (4) it is filtered off; PA0 (5) it is washed; PA0 (6) it is dried; and PA0 (7) it is then calcined at a temperature above the temperature for the formation of the desired garnet structure.
This process makes it possible to:
(a) obtain the garnet structure by calcination at a lower temperature and for a shorter time than in the processes of the prior art;
(b) obtain the garnet structure outside the enclosure for drawing the monocrystal, which ensures exact knowledge of the composition of the starting polycrystalline garnet by virtue of the possibility of characterizing the latter (for example, by measuring the crystal parameter); and
(c) achieve an absolutely complete reaction for the formation of the garnet structure; the polycrystalline garnet obtained is pure and, in particular, it does not contain unreacted oxides.
Thus, during the heating of the polycrystalline garnet in the drawing crucible, the evaporation of the subspecies of certain oxides is much less extensive than in the processes of the prior art. This results in the following additional advantages during the drawing of the monocrystalline garnet:
(a) Exact knowledge of the composition of the molten bath at the moment of drawing obviates the need to use empirical relationships aimed at correcting the various evaporations.
(b) Limitation of the formation of subspecies of certain oxides makes it possible to restrict the reactions of these subspecies with the drawing crucible, and this makes it possible to limit, on the one hand, the attack and the rapid destruction of the crucible, and, on the other hand, inclusions, in the drawn monocrystal, of the metal of which the crucible is made. These last advantages prove to be of considerable practical importance, especially in the particular case of drawing monocrystals of Gd.sub.3 Ga.sub.5 O.sub.12 from an iridium crucible.
However, although this process exhibits considerable advantages compared with the processes of the prior art, it does not prove totally satisfactory for obtaining polycrystalline garnets containing aluminium and/or gallium and/or indium and at least one element taken from the group comprising the rare earths and yttrium. In fact, for these garnets, completely quantitative co-precipitation can prove difficult to achieve and the filtration and washing operations can prove lengthy to carry out.
Applicants have invented a novel process for the manufacture of polycrystalline garnets of the foregoing types, which solve the above-mentioned problems, while at the same time, retaining the numerous advantages of the co-precipitation process. This is an important object of the present invention.
It is also an object of the present invention to provide novel processes for the production of polycrystalline garnets and the corresponding monocrystals.
It is a further object of the present invention to provide novel polycrystalline garnets and their corresponding monocrystals.
Other objects will be apparent to those skilled in the art from the present description, taken in conjunction with the appended drawings.